Pattern and change in an Acacia aneura shrubland and Triodia hummock grassland mosaic on rolling hills in central Australia

The environmental correlates of a mosaic of Acacia aneura F.Muell. ex Benth. shrublands and Triodia hummock grasslands on rolling conglomerate hills in central Australia were explored by indirect gradient analysis. A non-metric multidimension scaling ordination, based on the presence or absence of plant species, clearly separated the A. aneura shrubland from the Triodia hummock grassland; there were few intermediate quadrats. The A. aneura shrublands occurred on relatively deeper skeletal soils than the Triodia hummock grasslands. Unlike unidirection successional gradients from frequently burnt, treeless vegetation to very infrequently burnt forest vegetation, the ordination presented here is unusual because the quadrats with the greatest cover of A. aneura and Triodia occur on the poles, and quadrats from the most recently burnt vegetation and with the least cover of A. aneura and Triodia occur in the centre of the ordination. Interpretation of aerial photography taken in 1950 and 1987 showed that there has been no statistically significant change in the coverage of these two communities over that time period. Some quadrats that contained Triodia on the boundary of A. aneura shrublands had numerous charred A. aneura stumps which was interpreted as indicating some retreat of the shrublands. Experimental studies are required to determine the stability of the current pattern. Biogeographic evidence, however, shows that the spatial distribution of both communities has fluctuated at a continental scale through geological time.